Electron multiplier



p 26, 1939- G. A. MORTON 2,174,162

ELECTRON MULTIPLIER Filed. 001;. 25, 1935 l appended claims,

UNITED STATES PATENT orncs ELECTRON MULTIPLIER George A. Morton, Haddon Heights, N. J assignor to Radio Corporation of America, a corporation of Delaware Application October 25, 1935, Serial No. 46,644-

7 Claims.

This invention relates to the construction and operation of electron discharge devices, particularly to the provision of improvements in electron-lens systems for electrostatically controlled electron-multipliers or" the general type disclosed in copending application, Serial No. 8,630, to Morton and Flory, filed'l ebruary 28, 1935. Discharge devices of this type are adapted to be used as amplifiers, oscillators, modulators and frequency doublers. They are especially usefulas direct current amplifiers; radio frequency amplifiers and as self-contained, photo-actuated amplifiers.

The novel features characteristic of the invention are set forth with particularity in the will be best understood by reference to the accompanying drawing wherein:

Figure l is a diagrammatic representation of a bi-part electron-lens, designed in accordance wlth the principle of the invention, showing equipotential electro-static' lines of force and their focusing effect upon a family of electrons.

Fig. 2 is a View in perspective of a preferred embodiment of the invention, a portion of the tube envelope being broken away to show the elements more clearly. I

Fig. 3 is a diagrammatic view of the device shown in Fig. 2, exempliying the'manner in which the several electrodes are energized when the device is utilized for certain of the purposes to which it is adapted. j

If electrons moving with considerable velocity strike an electrode surface, secondary electrons are emitted. The numbcr'of secondary electrons which are liberated will depend, in part, upon the velocity of impact and upon the nature of the surface material of the electrode upon which the primary electrons impinge. If the newlyliberated electrons are thrown against another electrode the number of electrons may once more be increased by a useful factor, again and again, until the gain is, in fact, enormous,

The gain per stage will depend upon the field available to draw the several bundles or families of electrons away from their parent electrodes and also, in large measure, upon the ability to focus the electrons of a given family upon its terminal or target electrode without loss. To increase the efiiciency of uch devices, it has previously been proposed to provide conduits through which the electrons travel, the electrically charged Walls of the conduit serving, at least to some extent, to limit diffusion of the electrons.

Morton and Flory in their above identified copending application divide their conduits into separate sections spaced from the electrodes which they connect and maintain the several conduit sections and electrodes at different potentials whereby the electrons are not only The invention itself, however,

vide an electron-multiplier characterized by an economy of parts and operating potentials.

Another object of the invention is to provide an improved electron-focusing system, whereby a large number of multiplying stages may be contained within a single tube without loss of efficiency caused by electrons missing the targets,

The present invention contemplates and its practice provides an electron-lens or focusing system of the electro static type wherein, by reason of certain correlated dimensions of the separate conduit members, the electrons from one electrode or target are sharply focused onto a region on the succeeding target substantially equal in area to that on the electrode of origin,

l is illustrative of the principle by which the focusing action is produced. Two terminal electrodes are shown, an electrode which in this case may be considered a primary electron emitter of a desired type is designated 5 and the target electrode 3. A bi-part conduit comprising a pair of spaced hollow members 2 and d is laid endto-end intermediate these electrodes, part 2 preferably being integral with electrodes l and part i integral with electrode 3.--When so combined electrode 5 and its conduit section 2 operate at the same potential, electrode 3 and its section t preferably being maintained at a single higher potential. Parts 2 and i are conveniently of cylindrical construction and of the same diameter; the space 8 between these separated cylinders is not extremely critical but should ordinarily be no greater than, say, 3% of the total length of the assembly. These coaxial cylinders 2 and l and the-space s therebetween constitute an electron-lens which, as previously indicated serves to confine and to focus electrons emanating from the emitter 1 upon the electrode 3.

It has been found, in accordance with the invention, that if these cylinders 2 and i are so formed that the length of the one adjacent to the electron source is equal to substantially twothirds of its diameter, and the length of the one adjacent to the target is substantially twice that of the former (their diameters being the same) a sharp image of unity magnification is achieved irrespective of the exact potential applied to the second cylinder,

As indicated in Fig. 1 this correlation of dimensions ensures a distribution of the electrostatic lines of force f such that the electrons, leaving electrode l in random directions, are concentrated into a beam b and directed exactly to the central portion of the target 3.

Fig. 2'- shows an electron-lens system Within the present invention incorporated in a plural stage electron-multiplier of the photo-sensitive type. The device shown in the drawing comprises an evacuated envelope I0 conveniently in the form of an elongated cylindrical glass tube having a reentrant stem I2 and press [4 through which lead wires IG l8 20 and 22 extend. The electrode assembly comprises a photo-sensitive cathode IS, a first terminal or target electrode l8, a second target 20 and a collecting plate or anode 22. The lens intermediate the oathode l6 and target [8 is constituted by a pair of hollow cylindrical members [S and I8"; that between the first and second targets by cylinders l8 and 20 A short cylinder 20 extends from the second target 20 towards, but does not touch the output electrode 22. A long open ended cylinder l6 is effectively joined to the edge of cathode l6.

The lens cylinders leading to and away from each target are shown at an angle of 90 with respect to each other and the electrode surfaces are inclined at an angle of 45 with respect to the long axis of these cylinders; they may, however, be arranged in any suitable or convenient manner.

The dimensions of the lens cylinders are the same as in Fig. 1, i. e., the cylinder adjacent a given electrode on the side corresponding to the direction of electron travel has a length measured from the mid-section of the electrode which it encloses substantially equal to two-thirds of its diameter, and the length of the other cylinder is substantially equal to four-thirds its diameter measured from a similar point on the surface of the electrode to which the electrons are directed.

In operating the device of Fig. 2, the photosensitive cathode I6, the multiplying electrodes l8 and 20, anode 22 and their associated conduit members may be supplied with suitable potentials from any suitable source of direct current. This source is exemplified in Fig. 3 by a potential divider 30 to the negative terminal of which is connected the cathode I6 and to the positive terminal of which, through an output resistor 32, is connected the output electrode 22. Such mode of connection has been found to be advantageous but it is not to be inferred from the drawing that the: invention is to be limited by the specified circuit connections or by the applied potentials.

When light, either steady of fluctuating, from an external source 34, is directed as by a lens 36 through the cylinder I6 and falls upon the photo-sensitive cathode [6 primary electrons are emitted therefrom in random directions. By reason of the positive potential applied to the first multiplying electrode l8 the primary electrons are accelerated toward it and were it not for the electron lens constituted by the spaced hollow cylindrical members 16*, l8 would bombard the entire surface of this target. These cylinders, however, when constructed in accordance with the formula of the invention serve as a converging electron lens and cause the secondary electrons to be focused upon an area of the target |8 substantially no greater than the area of electron origin.

The secondary electrons liberated by impact from the electrode [8, in turn, are accelerated toward and focused upon the electrode 20, from which another and multiplied family of electrons originates. This last family of electrons is drawn to the output electrode 22 and gives rise to current in the output resistor 32.

Obviously, the electron-lens system of the present invention may be incorporated in a device having any desired number of terminal or target electrodes. It is likewise obvious that any suitable electron source, such as one of the thermionic type, may be substituted for the photosensitive cathode herein described. Other modifications will suggest themselves to those skilled in the art, therefore the invention is not to be limited except by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. An electron multiplier comprising an electron emissive cathode, a target electrode adapted to release secondary electrons by impact excitation and accessible to electrons from said cathode, a collector electrode accessible to secondary electrons from said target electrode, and an electron lens comprising a conduit substantially coextensive in length with the space between said cathode and target through which electrons from said cathode pass, said conduit comprising a pair of spaced hollow members laid end-to-end intermediate said electrodes, said hollow members being mechanically integral respectively with the electrode adjacent thereto, the length of one of said hollow members being less than its diameter and the length of the other of said hollow members being greater than its diameter.

2. The invention as set forth in claim 1 wherein said hollow members are respectively integral with the electrode adjacent thereto.

3. An electron-multiplier comprising a primary-electron emissive electrode, a secondaryelectron emissive electrode accessible to primaryelectrons from said first mentioned electrode, a target electrode accessible to secondary-electrons from said second mentioned electrode, a conduit of substantially uniform diameter defining the path of the electrons among said electrodes, said conduit comprising a plurality of pairs of hollow members, the members constituting each pair being laid end-to-end between adjacent electrodes and spaced one from another, the length of one hollow member of each pair being equal to substantially two-thirds the diameter of said conduit and the length of the other member of each pair being substantially twice that of said first-mentioned member.

4. The invention as set forth in claim 3 wherein the shorter of the hollow members of one pair is joined to the longer member of another palr.

5. The invention as set forth in claim 3 wherein one of the hollow members of one pair is joined at substantially a right angle to another member of another pair.

6. The invention as set forth in claim 3 where-- in certain of said electrodes are inclined at an angle of substantially 45 with respect to the long axis of said conduit.

7. An electron multiplier comprising a primary-electron emissive cathode, a plurality of secondary-electron emissive electrodes and a target electrode mounted in the order named in an evacuated container, and an electron-lens comprising a pair of spaced hollow members of different lengths laid end-to-end between said secondary-emissive electrodes and between said target electrode and the secondary-emissive electrode adjacent thereto, one of said secondaryemissive electrodes having one of the long and one of the short of said hollow lens members connected thereto.

GEORGE A. MORTON. 

